Method of providing static shielding of transformer windings

ABSTRACT

THE PRESENT DISCLOSURE RELATES TO A METHOD AND APPARATUS FOR PROVIDING IMPROVED STATIC SHIELDING OF TRANSFORMER WINDINGS. THE IMPROVED STATIC SHIELDING IS COMPRISED BY INSERTING SERIES CAPACITANCE BETWEEN THE WINDING TURNS IN A MANNER SUCH THAT THE VALUE OF A FIRST SERIES CAPACITANCE CS1 ON THE LINE OF THE TRANSFORMER IS RELATED TO THE VALUE OF A LAST SERIES CAPACITANCE CSN ON THE EARTH SIDE OF THE TRANSFORMER IN ACCORDANCE WITH THE EXPRESSION   WHEREIN K IS A CONSTANT HAVING A VALUE BELOW 0.05 AND CG1 IS THE CAPACITANCE TO EARTH OF ALL THE TURNS THE VALUES OF SERIES CAPACITANCE OF THE INTERMEDIATE TURNS BETWEEN SAID SERIES CAPACITANCE CS1 AND CSN AND ARE RELATED IN A   MANNER SUCH THAT SAID INTERMEDIATE VALUES VARY APPROXIMATELY LINEARLY BETWEEN THE VALUES OF SAID SERIES CAPACITANCES CS1 AND CSN.

Feb. 2, 1971 KENICHI OKUYAMA 3,560,902

` METHOD OF PROVIDING STATIC SHIELDING OF TRANSFORMER WINDINGS Filed June 14.` 19e? s sheets-sheet 1 NVENT( )R BY K l ATTORNEY Feb ,2, 1971 KENlcl-n oKuYAMA METHOD OF PROVIDING STATIC SHIELDING OF TRANSFORMER WINDING 3 Sheets-Sheet 2 Filed June 14. 19s? Osa NVENTOR Kflw CHI OKQYH/mq I' ATTORNEY Fel` --KENIH|'oKuYAMA 35605902 Milz-mono? PROVIDING {STAT-1c SHIELDING or TRANsFoRMEn wiNDING-s Filed Jungla `mes? 4 s sheets-sheet s -IIWENTUR y f mM/ffy United States Patent O 3,560,902 METHOD OF PROVIDING STATIC SHIELDING OF TRANSFORMER WINDINGS Kenichi Okuyama, Hitachi-shi, Japan, assignor to Hitachi, Ltd., Tokyo, Japan Filed June 14, 1967, Ser. No. 645,980 Int. Cl. H01f 15/14 U.S. Cl. 336-70 9 Claims ABSTRACT OF THE DISCLOSURE The present disclosure relates to a method and apparatus for providing improved static shielding of transformer windings. The improved static shielding is comprised by inserting series capacitance between the winding turns in a manner such that the value of a first series capacitance CS1 on the line side of the transformer is related to the value of a last series capacitance Csn on the earth side of the transformer in accordance with the expression Csi k \/Cg1 Cal wherein k is a constant having a value below 0.05 and Cgl is the capacitance to earth of all the turns the values of series capacitance of the intermediate turns between said series capacitance CS1 and Csn and are related in a manner such that said intermediate values vary approximately linearly between the values of said series capacitances CS1 and Csn.

BACKGROUND OF THE INVENTION The present invention relates` to a method and apparatus for providing improved static shielding of transformer windings.

Power transformers are directly connected to transmission and distribution lines and are directly affected by both external and internal abnormal voltages supplied thereto through the lines. Therefore, special consideration for said effect must be given with regard to electrical insulation between the windings in the design of the transformer. In order to minimize the effect of abnormal voltages, it has been acknowledged that means must be provided to obtain uniform distribution of oscillating potentials in the event of such undesired impulse voltages being applied across the transformer. Such means for obtaining uniform distribution of such oscillating potentials has been attained in the past by providing series capacitance Cs of the greatest possible value to thereby uniformalize the initial distribution of such impulse potentials. Such uniform initial distribution is determined primarily by the electrostatic capacitance between the turns and between the turns and ground. Thus, such means as high series capacitance windings as shown in U.S. Pat. No. 3,246,270 or electrostatic shielding windings as shown in U.S. Pat. No. 2,905,911 heretofore have been proposed. In such prior art structures, however, the values of series capacitance Cs required to establish uniform capacitance from the line side to the earth have inevitably resulted in transformer windings of greater volume and higher cost.

The inventor has made a reinvestigation with respect to the relation between the series capacitance and the initial distribution of potential across transformer windings upon occurrence of impulse voltages. As a result thereof, it has been discovered that the especially great electrostatic capacitance values `Cs employed in the prior art structures are not necessarily required to attain uniform initial distribution of impulse potentials. Or more precisely, even relatively small series capacitance will be effective to sufiiciently uniformalize the initial distribution of potential when an arrangement is made so that the series capacitance Cs are made to have successively smaller values at portions closer to an earth terminal of a transformer (as opposed to the line terminal) as will be described hereinafter. Also, it has been discovered that by thus uniformalizing the initial distribution of potential, the potential distribution in the event of occurrence of internal oscillation can also be made uniform.

According to the invention, there is provided a new and improved static shielding construction for transformer windings wherein the value of a first series capacitance CS1 on the line side of transformer winding is first determined, then the value of a last series capacitance CSn on the earth side of the transformer is related thereto in accordance with the expression.

wel

CE1 OEI wherein k is a constant having a value below 0.05 and Cgl is the capacitance to earth of the windings, and the values of the series capacitance between the intermediate windings between said series capacitances CS1 and Csn are related in a manner such that said intermediate values vary approximately linearly between the values of said series capacitances CS1 and Csm.

It is a primary object of the invention therefore to provide an improved static shielding construction for power transformer windings whereby power transformers of compact structure are made available wherein the volume occupied by shielding windings can be reduced to a minimum.

Other objects, features and other attendant advantages of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become better understood from the following detailed description considered in connection with the accompanying drawings; wherein the same reference character is employed in each of the several figures to identify the same element; and wherein:

FIG. 1 is an equivalent circuit of a known transformer winding construction;

FIG. 2 is an equivalent circuit of the transformer winding shown in FIG. 1 in the event of impression of impulse voltage thereon;

FIG. 3 is a graphic illustration of several modes of initial distribution of potential under various arrangements of series capacitance in a transformer;

FIG. 4 is an enlarged view of a pair of windings on the line side of a transformer illustrating the elemental structure of the present invention;

FIG. 5 is an equivalent circuit of the winding construction shown in FIG. 4;

FIG. 6 is an equivalent circuit of a transformer winding according to the invention including series capacitance grouped into a plurality of blocks;

FIG. 7 is a graphic illustration showing the comparison between the initial distribution of potential according to the invention and the initial distribution of potential in a conventional transformer winding;

FIG. 8 is a diagrammatic sectional view of the winding turns of a new and improved transformer constructed in 3 accordance with the present invention in the manner of FIG. 6;

FIG. 9 is an equivalent circuit of the speciiic winding construction shown in FIG. 8; and

FIG. 10 is a diagrammatical view of the windingy turns of still another transformer constructed in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, there is shown an equivalent circuit of a known transformer winding construction which includes inductances L, a plurality of series capacitances CS1, CS2

. Csk Csn and parallel capacitances Cgn, Cgl

gk Cgn between a terminal S on the line side and a terminal E on the earth side. Symbols i1 ik in indicate currents flowing through unit circuits comprising Cgo, C51 and Cgl Cgk 1, Csk and Cgk and Csm respectively.

When now an impulse voltage is impressed on the circuit as shown in FIG. l, the inductances L of the transformer winding provide great reactance which brings forth a property very analogous to an open circuit, while, the electrostatic capacitances provide extremely low reactance. Therefore, it may be considered that the potential distribution is determined solely by these electrostatic capacitances Cs and Cg. In this case, the circuit of FIG. l can be expressed as an equivalent circuit as shown in FIG. 2, and the initial potential distribution is determined by such circuit. The initial potential distribution when a voltage v(t) is impressed on the circuit of FIG. l is equal to the initial potential distribution when a unit voltage is impressed on the circuit of FIG. 2. Now,

supposing that Ik=fl'kdl 1 the following equation can be obtained\for the circuit of FIG. 2.

. ogm1 As described in the foregoing, the initial potential distribution in the circuit of FIG. l equals the distribution when the unit voltage is impressed on the circuit of FIG.,2. Therefore, Krk in the case of impression of the unit voltage can be obtained by inserting a value of v(t)=1 in the Equation 2. Then, the initial distribution relative to ground in the circuit of FIG. l can be obtained from terminal voltage vk of the respective parallel capacitances Cg of FIG. 2, and the following equation is given.

kk k+1 Ogk Calculations are made on the basis of these Equations 2 and 3 by varying the arrangement of the series capacitances Cs, and as a result thereof, various initial potential distributions can be obtained as shown in FIG. 3. In the calculations and in later description, it is assumed that all of the parallel capacitances Cgl Cgk Cgn have the same value.

According to the invention, it has been found out that the initial potential distribution can most reasonably be uniformalized when the following equation is utilized for proper arrangement of the series capacitances Cs.

OSD: Cal Cgi wherein, k is a constant having a value less than 0.05.

In order to provide an (optimum) arrangement of the series capacitances Cs for uniformalizing the `initial potential distribution, Csl is set at a suitable value at first, and then Csn can be obtained from the Equation 4. Then, values of CS2 Csk Csn 1 lying between CS1 and CSn are so determined as to approximately linearly vary between CS1 and Csn. Thus, it will be understood that a space occupied by a shielding winding in the transformer winding can be reduced by providing the reasonable arrangement of the series capacitance Cs on the basis of the Equation 4, without making all of the series capacitances Cs have a great value as has been the case in the, conventional method and apparatus. Further, it will be apparent that, owing to such arrangement, the potential distribution in the event of occurrence of internal oscillation can also be made uniform.

FIG. 4 is an enlarged view of a pair of windings on the line side illustrating the principle and the elemental structure of the present invention, and FIG. 5 is an equivalent circuit of the structure shown in FIG. 4. Reference numerals 1-10 and 201-209 designate main conductors and static shielding conductors, respectively.

The interturn capacitance of the winding structure shown in FIG. 4 in which static shielding conductors are wound between main conductors, i.e., the equivalent series capacitance Cs, is determined by where, N is the number of turns of the static shielding conductors and Cc is the electrostatic capacitance between one static shielding conductor and an adjacent main conductor. The electrostatic capacitance Cc is determined from the space between and the facing area of the static shielding conductor and the adjacent main conductor. The space 'between the static shielding conductor and the adjacent main conductor can be adjusted by changing thickness of the insulating layer surrounding the static shielding conductor or the adjacent main conductor. On the other hand, the facing area of the static shielding conductor and the adjacent main conductor can be adjusted by changing longitudinal length or width of the static shielding conductor. Therefore, to obtain a substantially linear distribution of the series capacitance from the line side to the earth side according to the present invention, the following determinants may be appropriately adjusted. For example, either the number of turns of the static shielding conductors N, the space between the static shielding conductor and the adjacent main conductor, the facing area of a main conductor and an adjacent shielding conductor, or combinations of such determinant may be varied as the case may require.

From the above mentioned variable determinants of the series capacitance Cs, it will be understood that values of the series capacitance may be changeable gradually from the line side to the earth side in accordance with the concept of this invention. However, in a preferred embodiment of the invention, all of the series capacitances C51 CS1.: Csn may not have different value s from one another, and the circuit may be divided into a plurality of blocks wherein an arrangement is made so that the series capacitances in any one block have the same value. That is to say, it may be extremely convenient for the purpose of manufacture if these capacitances may be grouped aS CS1, CS1, CS1, CS1 Csk, Csk, Csk Csm Csn, Csn as shown in FIG. 6.

FIG.7 shows the initial potential distribution obtained from a structure including the series capacitances C51 Csk Csn which are grouped into three blocks. Three curves are shown wherein the curve A shows the initial potential distribution when the entirety of CS/Cg is made to have a value of 100. The curve B shows the initial potential distribution when the entirety of Cs/Cg is made to have a value of 400. The curve C shows the initial potential distribution when k equals 0.05 in the Equation 4, and values of C51/ Cgl in the first block, Clsg/Cgl in the last block, and CS2/Cg1 in the intermediate block are made as 100, 25 and 56.25, respectively. It will be apparent from the curves shown that the curve C is very close to the curve B having Cs of the extremely great value, and thus the initial potential distribution of sufficiently uniform nature can be obtained.

FIG. 8 is a diagrammatic sectional view of the windings of a new and improved transformer constructed along the lines proposed with relation to FIG. 6, and FIG. 9 is an equivalent circuit of the winding construction shown in FIG. 8.

In FIG. 8, there is shown an embodiment constructed in such manner that the number of turns of static shielding conductors N are varied from one block of turns to another.

Referring now to FIGURE 8, reference I indicates an iron core, WL is a low voltage winding illustrated in block diagram form only, and WH is a high voltage winding to which the present invention has been applied. The high voltage winding WH is comprised by a plurality of layers of disk-like coils having a line side terminal t1 and an earth side terminal t2. The reference numerals 11 through 94, respectively, indicate main conductors and reference numerals 211 through 293, respectively, indicate static shielding conductors each wound between the main conductors. As is clear from FIGURE 8, the static shielding conductors from the first layer to the sixth layer on the line side are wound in a manner such that there are four turns between the main conductor, those from the seventh layer to the twelfth layer are wound so that there are three turns, and those from the thirteenth layer to the fourteenth layer provide two turns.

FIGURE 9 is an equivalent circuit diagram of FIG- URE 8 wherein L represents the reactance of a unit winding defined by two corresponding disk like layers of each disk coil, Cg is the capacitance to earth of the coil and CS1, CS2 and C53 are the respective series capacitances between the coil elements. These capacitances are related as set forth in the following equations:

Ca2 Cel The embodiment of the invention shown in FIGURES 8 and 9 corresponds to the equivalent circuit shown in FIGURE 6 of the drawings. In addition to fabrication in in the above manner, continuous variation of the series capacitance Cs from the line side to the earth side of the transformer can be carried out in accordance with the present invention by any of the known techniques of transformer fabrication. For example, the method of fabrication described above could be carried' out either by winding the static shielding conductors not in integral numbers but, for example, by fractional steps such as 4, 3.8, 3.5, 3.3, 3 turns as would be obvious to one skilled in the transformer art, or by modifying the thickness of the insulating layer of the static shielding conductor as will be described hereinafter.

As stated in connection with the above embodiments, it is understood that the unit capacitance Cs is comprised by two disk-coils (for example Al-Az as shown in FIG- URE 8) but this invention can also be applied when the unit capacitance is composed of any number of disk-like coil layers, etc. in ways analogous to those described above.

The series capacitance Cs may also be adjusted by modifying the connection of the static shielding conductors as illustrated in FIGURE l0 in contrast to varying as above mentioned, the number of turns of the shielding conductors 611, 612 the spacing between the two adjacent conductors by a shielding conductor or the facing area of a main conductor and an adjacent shielding conductor, etc.

In the winding structure shown in FIGURE 8 each pair of adjacent turns of the corresponding disk-like coil layers (such as 21 and 22) are so arranged as to have a potential equal to that of the main turns (such as 22- 23) of the disk-like coil layers adjacent said corresponding disk-like coil layers, while here in FIGURE l0 each pair of adjacent turns (such as 11-12) are so connected as to have a potential equal to that of main turns (such as 11-33) of the disk-like coil layer spaced apart as far as one pair of disk-like coil layers from the disk-like coil layer which includes the pair of turns under consideration.

Assuming that the conditions under which the static shielding windings are made are the same in the winding structures shown in FIGURES 8 and 10, the equivalent series capacitance Cs of the structure shown in FIGURE 10 amounts to 4 times that shown in FIGURE 8. Although spaced apart as far as one pair of disk-like coil layers is mentioned above, one may have a greater series capacitance by an arrangement in which the pair of disklike coil layers including the pair of main conductors are spaced apart as far as more than two pairs of disk-like coil layers from the disk-like coil layer which is connected to be at an equal potential to the pair of main turns.

In the winding structure of FIGURE 10, since the potentials at points p and q are substantially equal to those at points p and q respectively, it is also possible, without making a connection between points p and p' and between q and q', to obtain a series capacitance equal to that obtained `when such connections are made as shown in FIGURE l0. Thus, it can be understood that the equivalent series capacitance may also be varied within a wide range by modifying the manner of connection of the static shielding conductors.

Although the embodiments described hereinabove employ shielding conductors wound between main conductors whereby the series capacitance Cs is adjusted, it will be readily understood by those skilled in the art that where no static shielding conductor is provided the series capacitance can also be adjusted by modifying the connection of the coil windings of each of coil layers in a manner, for example, such as shown in British Pat. No. 587,997.

From the foregoing description, it will be appreciated that the invention is quite effective to reduce substantially the size and cost of transformers by improving the winding construction, and is very advantageous from the viewpoint of economy, operation and ease of handling.

It is therefore to be understood that variations and modifications of this invention will be apparent to those skilled in the art in the light of the above teachings and that any such modifications and variations are considered to be `within the scope of the present invention as defined by the appended claims.

I claim:

1. A method of manufacturing transformers having improved distribution of oscillating potentials in the event C gn CSI =k C g1 Cel wherein k is a constant having a value below 0,05 and CS1 is the capacitance to earth of all the turns and the values of the series capacitances of intermediate turns between said series capacitances CS1 and CSn are related in a manner such that said intermediate values vary approximately linearly between the values of said series capacitances lCS1 and CSn.

2. A method of manufacturing transformers having improved distribution of oscillating potentials in the event of an impulse voltage being applied across the transformer windings comprising introducing series capacitance between the transformer winding coils in a manner such that a plurality of blocks of coils are formed with each of the coils in each of the blocks having series capacitance of substantially the same value, the value of the first series capacitance CS1 of the coils in the first block on the line side being related to the value of the last series capacitance CSn of the coils in the last block on the earth side in accordance with an equation Og] Cgl wherein k is a constant having a value below 0.05 and CS1 is the capacitance to earth of the coils, and the values of series capacities of intermediate blocks of coils between said series capacitances CS1 and Csn are related in a manner such that said intermediate values vary approximately linearly between the values of said series capacitances CS1 and CSn.

3. A transformer comprising winding means and means for introducing series capacitance between unit portions of said winding means in which the first unit portion of said winding means on the line side has a series capacitance the value of which is CS1, the last unit portionthereof on the earth side has a series capacitance the value of which is CSn and is related to CS1 by the expression:

Jn@ C. C..r

wherein k is a constant below 0.05 and Cg is anaverage value of the capacity to earth of the unit portions, and the respective unit portions of said winding means between said first unit portion and said last unit portion have series capacitance values which vary substantially linearly between the values CS1 and CSS.

4. Atransformer according to claim 3 wherein the means for introducing series capacitance between unit portions of the winding means is comprised by static shielding windings.

5. A transformer according to claim 4 wherein the space between the static shielding conductor and the adjacent main conductor is varied from the line to the earth side of the transformer.

6. A transformer according to claim 4 wherein the area of the static shielding winding confronting the main winding turns is varied from the line to the earth side of the transformer.

7. A transformer according to claim 4 wherein the manner of interconnection of the static shielding windings is varied from the line side to the earth side of the transformer.

8. A transformer comprising winding means including a plurality of blocks of coils, and means for introducing series capacitance between the blocks of coils in thewinding means, the first block of coils on the line side having an average value CS1 of series capacitance between the coils which is related to an average value CSn of the series capacitance in the last block of coils on the earth side by the expression:

Can Cal VCS kCg 0 CS1 and CSn.

References Cited UNITED STATES PATENTS 12/1964 Bedil 336-69 4/1966 Stein 336-70 THOMAS l. KOZMA, Primary Examiner 

